1. Field of the Invention
This invention relates to multiplex signal transmission systems and, more particularly, to the packet transmission of speech on such systems.
2. Description of the Prior Art
It is well known in the art how to use a plurality of transmission channels for transmitting speech signals from a larger plurality of signal sources. The technique is commonly called Time Assignment Speech Interpolation (TASI). In such a system, speech signals in the form of talkspurts are detected and transmitted, as they occur, over any idle and available transmission channel of the plurality. Since normal human speech includes a considerable number of silent intervals between words, phrases, and sentences, as well as silent intervals while listening to the other party, a considerable efficiency can be obtained in the use of the transmission facilities. Indeed, the ratio of the number of speech sources to the number of transmission channels (the TASI advantage) falls between two and three. One such TASI system is shown in A. R. Kolding et al U.S. Pat. No. 2,957,946, granted Oct. 25, 1960. Another type, a "one-man" TASI system, is described in J. L. Flanagan U.S. Pat. No. 3,158,693, granted Nov. 24, 1964.
It is also well known how to transmit coded information on common transmission facilities by assembling such information into packets which compete for common transmission facilities on a first come, first served, basis. Such packets normally include header information identifying the destination of the packet and, in some cases, routing information. Such packet transmission systems have, in the past, been restricted primarily to digital data rather than speech information due to the inability to guarantee delivery of all transmitted signal packets in the correct order and with tolerable delay. One such system is disclosed in J. R. Pierce U.S. Pat. No. Re. 28,811, reissued May 11, l976.
One of the limiting factors on the utilization of time assignment speech interpolation systems is the necessity to acquire and connect a new transmission channel virtually at once upon the initiation of a new talkspurt. This requirement restricts the loading which can be placed on the common transmission channels to insure the availability of an idle channel for a very large percentage of time. The requirement for always providing an available channel limits the number of speech signal sources which can be connected to the common channels.
Packet transmission systems, on the other hand, assume that all packets will be delivered to their intended destination eventually and that the exact timing of the packet deliveries is not essential to the proper reception of the packets. It is the receiver's job to sort the received packets into the correct temporal order before reconstituting the signal. Such packet transmission systems also require local buffering of the packets during completion for the common transmission facilities and until assignment of the facilities to that particular packet.